1. Field of the Invention
The present invention relates to a hybrid construction machine configured to make a regenerative action through return fluid from a hydraulic actuator such as a boom cylinder.
2. Description of the Background Art
The background art of the present invention will be described by taking an excavator shown in FIG. 7 as an example.
This excavator comprises: a crawler-type lower traveling body 1; an upper slewing body 2 mounted on the lower traveling body 1 slewably about an axis O perpendicular to a ground surface; an attachment AT attached to the upper slewing body 2; an engine; and a hydraulic pump configured to be driven by the engine. The attachment AT includes a boom 3, an arm 4, a bucket 5, and a plurality of hydraulic actuators, namely, a boom cylinder 6, an arm cylinder 7 and a bucket cylinder 8. The lower traveling body 1 is configured to be driven by a traveling drive unit using a traveling hydraulic motor as a drive source, and the upper slewing body 2 is configured to be driven by a slewing drive unit using a slewing hydraulic motor as a drive source. The attachment AT is configured to be actuated by hydraulic fluid supplied from the hydraulic pump.
In the case of a hybrid type, the above excavator is further equipped with a generator motor configured to make a generator action and a motor action, and an electrical storage device, wherein the generator motor and the hydraulic pump are connected to the engine. The generator-motor is operable to charge the electrical storage device through the generator action, and to be driven by electric power stored in the electrical storage device, on a timely basis, to make the motor action, thereby assisting the engine.
As such a hybrid excavator, there has been known one type configured to make a regenerative action through return fluid from a hydraulic actuator, e.g., of the energy of return fluid from the boom cylinder 6 being subject to a boom-lowering operation, and to charge an electrical storage device with regenerated-electric-power generated through the regenerative action, as disclosed in JP 2006-336306A (Patent Literature 1). The Patent Literature 1 discloses making a slewing regenerative action during a slewing operation in addition to a boom-lowering regenerative action during the boom-lowering operation; more specifically, it is disclosed that, during slewing braking, the electrical storage device is charged with electric power regenerated by a slewing motor acting as a slewing drive source while regenerative braking force is generated.
There has also been known another type which comprises a slewing drive source composed of a hydraulic motor to which a generator is coupled to make a slewing regenerative action in the same manner as above, as disclosed in JP 2009-127643A (Patent Literature 2).
In the above heretofore known techniques, when a regenerative action through return fluid from a hydraulic actuator is made in a situation where the electrical storage device has a relatively high charging rate, the electric power generated through the regenerative action brings the electrical storage device into so-called “overcharge” beyond a proper charging rate, and the overcharge may involve a failure and life-shortening of the electrical storage device. Particularly, during an operation of lowering the boom 3 of the excavator as shown in FIG. 7, a possibility of overcharge increases, because return fluid is flowed out of the boom cylinder 6 at a large flow rate to allow larger regenerated-electric-power, specifically, larger boom-lowering regenerated-electric-power, to be generated. Besides, in a hybrid construction machine where both the slewing regenerative action and the boom-lowering regenerative action is made, not only the possibility of overcharge is further increased during a combined slewing and boom-lowering operation in which the two regenerative actions are simultaneously performed but also slewing braking performance may be deteriorated due to difficulty in regeneration of the entire slewing energy.
As means to prevent the overcharge, there could be consuming an unnecessary part of regenerative power through a regeneration resistor; however, this involves energy loss by throwing out regenerated-electric-power in the form of heat, and requires addition of special equipment including a relatively large-sized resistor for consuming the regenerated-electric-power as the regeneration resistor, a control device for the resistor, and further a device for treating heat generated from the regeneration resistor. This brings device layout especially in an excavator having severe restrictions in terms of space brings into difficulty and involves a significant increased cost. Using a large-capacity electrical storage device hard to overcharge also involves the same disadvantages as above, in terms of cost and installation space.